1. Field of the Invention
The present invention relates to an exposure apparatus, and the fabrication method of a semiconductor device using the same. More particularly, the present invention relates to the alignment of a mask with a wafer in a projection exposure apparatus having a projection optical system mounted thereon, which is a catadioptric type including a reflecting member.
2. Related Background Art
Conventionally, a projection exposure apparatus has been used for fabricating a semiconductor device, a flat panel display device (e.g., a liquid crystal display device), and so on in a lithography process. With a great increase made in the progress speed of a semiconductor technology in recent years, a progress in a microfabrication technology has been remarkable. For example, in a semiconductor device having a memory enabling writing/reading to be performed as occasion demands, i.e., Dynamic Random Access Memory (DRAM), currently a 16 M bit DRAM is a mainstream, but developments are under way to increase the degree of integration to the range of 256 M bit DRAM.
With such integration of the semiconductor device, an exposure wavelength has been made shorter than a currently mainstream i line (365 nm). In other words, a KrF excimer laser (oscillation wavelength 248 nm) has already been put to practical use. In addition, developments are under way to realize the practical use of an ArF excimer laser (oscillation wavelength 193 nm).
The shorter exposure wavelength causes a reduction in the transmittance of an optical material, and a great limitation is placed on the kinds of optical materials used for, for example a projection optical system. Consequently, in the conventionally mainstream exposure apparatus having the projection optical system of a catadioptric type, inconvenience inevitably occurs, i.e., the axial (longitudinal) chromatic aberration of the projection optical system becomes relatively large.
On the other hand, in the case of the exposure apparatus having the projection optical system of a catadioptric type, it is possible to suppress the axial chromatic aberration of the projection optical system even for the optical materials of limited kinds. However, in the projection optical system of the catadioptric type, because its optical axis is bent by the reflecting member without being linearly extended, inconvenience is inherent, i.e., the fluctuation of a mask pattern image (positional shift or rotational shift of an image) caused by the position fluctuation of the reflecting member occurs far more easily than that in the projection optical system of the refractive type. The projection optical system of the refractive type means a projection optical system including no reflecting members but a refraction system like a lens. The projection optical system of the catadioptric type means a projection optical system including at least one reflecting member and a refraction system like a lens.
Each of Japanese Patent Laid-Open No. 63-41023 and Japanese Patent Laid-Open No. 7-22350 disclose the system of alignment between a mask and a wafer (photosensitive substrate) in an exposure apparatus having the projection optical system of a catadioptric type mounted thereon. In the exposure apparatus disclosed in each of these publications, a beam splitter like a half prism is added in an optical path between the mask and the wafer, and the positions of the mask and the wafer relative to each other are measured by making an alignment light incident on the half prism.
Japanese Patent Laid-Open No. 5-21314 discloses a method of measuring the positions of a mask and a wafer relative to each other before exposure by using a reference mark formed on a wafer stage. This method is applied to the exposure apparatus of a scanning type (step-and-scan system) for transferring a mask pattern to each exposure area of the wafer while moving the mask and the wafer relative to each other with respect to the projection optical system of a refractive type.
In the exposure apparatus disclosed in each of Japanese Patent Laid-Open No. 63-41023 and Japanese Patent Laid-Open No. 7-22350, the half prism is added in the optical path between the mask and the wafer. In other words, in the optical path between the mask and the wafer, the half prism, not an essential component of the projection optical system, is disposed in surplus. Consequently, the optical performance of the projection optical system is adversely affected easily by the added half prism. In addition, since the optical separation surface (i.e., dividing surface of wave front) of the half prism cannot be corrected after its fabrication, excessively high accuracy is required for the fabrication of the half prism to prevent the optical performance of the projection optical system from being adversely affected.
On the other hand, in the case of the exposure apparatus described in Japanese Patent Laid-Open No. 5-21314, the positions of the mask and the wafer relative to each other are measured before exposure by using the reference mark formed on the wafer stage. In other words, the positions of the mask and the wafer relative to each other cannot be measured during exposure as occasion demands. Consequently, if the alignment system described in this publication is applied to the exposure apparatus having the projection optical system of the catadioptric type, even when the position of the reflecting member in the projection optical system changed during exposure and causes a mask pattern image to be shifted in position or rotation from a reference image-forming position, alignment between the mask and the wafer cannot be carried out for the projection optical system during exposure according to the positional or rotational shift of the mask pattern image as occasion demands.
The present invention is made with the foregoing problems in consideration. The object of the present invention is to provide an exposure apparatus capable of performing alignment between a mask and a wafer even during exposure as occasion demands, without providing any surplus optical members in an optical path between the mask and the wafer, and according to the positional or rational shift of a mask pattern image caused by the position fluctuation of a reflecting member disposed in the projection optical system of a catadioptric type. Another object of the present invention is to provide a fabrication method of a semiconductor device using the exposure apparatus.
In order to solve the foregoing problems, in accordance with a first aspect of the present invention, an exposure apparatus is provided, comprising: an illumination optical system for illuminating a mask having a specified pattern formed thereon; a projection optical system for projecting an image of the pattern on the mask to a photosensitive substrate through at least one reflecting member; a detection system for detecting an amount of fluctuation from the reference position of the at least one reflecting member; an arithmetic system for computing an amount of correction regarding at least one of the mask and the photosensitive substrate, to substantially align the pattern image formed in the state of being moved from a reference image-forming position with the photosensitive substrate, based on the amount of fluctuation detected by the detection system; and a driving system for moving at least one of the mask and the photosensitive substrate based on the amount of correction computed by the arithmetic system.
According to the preferred embodiment of the first aspect of the present invention, the arithmetic system computes the amount of positional shift and the amount of rotational shift of the pattern image from the reference image-forming position based on the amount of fluctuation detected by the detection system, and then computes the correction amount of the mask necessary for substantially correcting the amount of positional shift and the amount of rotational shift. The driving system moves only the mask based on the amount of correction computed by the arithmetic system. Alternatively, the arithmetic system computes the amount of positional shift and the amount of rotational shift of the pattern image from the reference image-forming position based on the amount of fluctuation detected by the detection system, and then computes the correction amount of the photosensitive substrate necessary for substantially aligning the photosensitive substrate with the pattern image formed in a state where at least one of the amount of positional shift and the amount of rotational shift occurs. The driving system moves only the photosensitive substrate based on the amount of correction computed by the arithmetic system. Otherwise, the arithmetic system computes the amount of positional shift and the amount of rotational shift of the pattern image from the reference image-forming position based on the amount of fluctuation detected by the detection system, then computes the correction amount of the mask necessary for substantially correcting one of the amount of positional shift and the amount of rotational shift, and still yet computes the correction amount of the photosensitive substrate necessary for substantially aligning the photosensitive substrate with the pattern image in a state where the other of the amount of positional shift and the amount of rotational shift occurs. The driving system moves the mask and the photosensitive substrate based on the correction amounts of the mask and the photosensitive substrate computed by the arithmetic system.
In accordance with a second aspect of the present invention, an exposure method is provided to project and expose a pattern image provided on a mask onto a workpiece through a projection optical system having at least one reflecting member.
In this case, the exposure method comprises: a detection step of detecting a fluctuation amount of the at least one reflecting member from a reference position; an arithmetic step of computing an amount of correction regarding at least one of the mask and a photosensitive substrate, necessary for substantially aligning the photosensitive substrate with the pattern image formed in a state of being moved from a reference image-forming position, based on the amount of fluctuation detected in the detection step; an alignment step of performing alignment between the mask and the photosensitive substrate with respect to the projection optical system by moving at least one of the mask and the photosensitive substrate based on the amount of correction computed in the arithmetic step; and an exposure step of illuminating the mask by an illumination optical system in a state where the mask and the photosensitive substrate are aligned with each other with respect to the projection optical system in the alignment step, and exposing a pattern of the mask on the photosensitive substrate through the projection optical system.
According to the preferred embodiment of the second aspect of the present invention, the detection step, the arithmetic step and the alignment step should preferably be performed before the exposure step. The detection step, the arithmetic step and the alignment step should preferably be performed during the exposure step as occasion demands. The exposure method should further comprise, preferably, a determination step of determining whether the fluctuation amount of the at least one reflecting member from the reference position is permissible or not and, if non-permissibility thereof is determined, then it is preferable that the arithmetic step and the alignment step are performed. The exposure apparatus should further comprise, preferably, a determination step of determining whether the fluctuation amount of the at least one reflecting member from the reference position is permissible or not and, if non-permissibility thereof is determined, then it is preferable that information regarding the amount of fluctuation is displayed. In addition, in the arithmetic step, an amount of correction should preferably be computed for at least one of the mask and the photosensitive substrate, which is necessary for substantial alignment between the mask and the photosensitive substrate with respect to the projection optical system. The computation is performed based on the movement amount of the pattern image from the reference image-forming position, which is obtained based on the amount of fluctuation detected in the detection step, and based on the position information of a mask stage for holding the mask and movable with respect to the projection optical system, and based on the position information of a substrate stage for holding the photosensitive substrate and movable with respect to the projection optical system.
Furthermore, in the first and second aspects of the present invention, the at least one reflecting member should preferably be a reflecting mirror having no power (refracting power).